Heretofore, as disclosed in the specification of Japanese Patent No. 2784154, a flow controller is constituted from a flow rate detector for measuring the flow rate of a fluid, and a proportional valve disposed in parallel with the flow rate detector. A main flow passage extends through the interior of the flow rate detector. On the inner wall of the main flow passage, a conduit inlet and a conduit outlet are opened, which are connected respectively to a conduit. A pair of heat-sensitive coils are wound on the conduit, which are connected to an amplifier. In addition, the flow rate of the fluid that flows through the conduit is estimated using a difference in resistance, which occurs due to a temperature difference generated between the heat-sensitive coils.
Further, in the proportional valve, a diaphragm is disposed in the center of a hollow proportional valve body, the periphery of the diaphragm being fixed to the proportional valve body. A valve stem, which interconnects the diaphragm and the valve body, is connected to the center of the diaphragm. In addition, a return spring is disposed on an upper portion of the diaphragm. The diaphragm is normally urged downward by the return spring, and together therewith, air pressure is supplied into a chamber beneath the diaphragm under a switching action of a supply solenoid valve, or alternatively, the air pressure inside the chamber is discharged to the exterior under a switching action of an exhaust solenoid valve.
The diaphragm is displaced upwardly in opposition to the elastic force of the return spring, whereby the valve body separates away from the valve seat enabling the fluid to flow therethrough. At this time, the flow rate of the fluid is detected by the flow rate detector, and based on the detection result detected by the flow rate detector, the flow rate is feedback controlled by operating the supply solenoid valve and the exhaust solenoid valve.
In general, with the aforementioned flow controller, because the structure thereof is complex and the apparatus is comparatively large in scale, recently, there have been demands for a structure that is simplified and smaller in scale.
On the other hand, with the conventional technique according to Japanese Patent No. 2784154, in the aforementioned flow rate detector, although a capillary heating system is adopted in which the heat-sensitive coils are wound with respect to a thin metal conduit, because a time lag is generated in the conduit when heat is transferred from the heat-sensitive coils, the response time is delayed. Furthermore, when the flow rate detector is assembled, since work is required to wind the heat-sensitive coils on the conduit and for welding the conduit with respect to the body, the assembly operation is complex, along with concerns over an increase in manufacturing costs.
Further, the proportional valve is constructed to carry out opening and closing of the valve body by a diaphragm, and in order for the proportional valve to be placed in a valve closed state in which the valve body is seated on the valve seat, a large elastic force is required for the return spring. As a result, there is a necessity for the return spring to be made large in scale, leading to a problem in that the product size is increased. Further, in the case that the elastic force of the return spring is large, the minimum operating pressure must also be large, and thus there is a concern that the proportional valve cannot be operated at low pressures.
Further, for example, in the case that a solenoid valve, which is operated by a control signal, is disposed in the proportional valve in place of a diaphragm operated by air pressure, and a structure is provided therein for opening and closing the valve body by operating the solenoid valve, then power consumption is increased, and together therewith, cases can be expected in which accurate detection results cannot be obtained, as a result of heat generated at the solenoid section of the solenoid valve being transferred to the flow rate detector.